Recovery system for recovering hydrocarbon vapor and offering improved stability

ABSTRACT

A recovery system for recovering hydrocarbon vapor given off by motor vehicle tanks while they are being filled with liquid fuel via at least one dispensing pipe. According to the invention, said recovery system comprises a recovery pipe associated with said dispensing pipe, a power pump placed on said recovery pipe, and a metering pump driven by a motor at an angular velocity that is substantially proportional to the volume flow rate of liquid fuel delivered by the dispensing pipe, said metering pump being mounted in series with said power pump via a differential pressure expander-regulator suitable for maintaining a pressure difference that is small between the outlet and the inlet of the metering pump so as to obtain a vapor volume flow rate that is substantially proportional to the angular velocity imparted by said motor. Application to dispensing liquid fuel.

The present invention relates to a recovery system for recoveringhydrocarbon vapor given off by motor vehicle tanks while they are beingfilled with liquid fuel via at least one dispensing pipe.

BACKGROUND OF THE INVENTION

In gas stations delivering liquid fuel, each of the dispensing pipes isterminated by a hose pipe equipped at one end with a gun that the userinserts into the feed pipe of the tank of the vehicle.

Naturally, as the tank fills, an equivalent volume of hydrocarbon vaporthat it contains escapes to the outside via said feed pipe. Inprinciple, vapor recovery thus consists in sucking up at the outlet ofthe feed pipe a volume of vapor that must at all times be equal to thedelivered volume of liquid fuel. For this purpose, the dispensing gun isequipped with a sleeve which penetrates into the outlet orifice of thetank of the vehicle, and into which the upstream end of a recovery pipeopens out, on which recovery pipe a suction pump is placed which returnsthe recovered hydrocarbon vapor to the storage tank for storing theliquid fuel. For the recovery system to operate satisfactorily, it mustbe possible to vary the speed of rotation of the pump so that theinstantaneous volume of sucked-up vapor equals the instantaneous volumeof dispensed liquid.

With known recovery systems, of the type including a vapor suction pumpwith the flow rate being regulated by varying its speed of rotation, itis difficult for the volume flow rate of the liquid being supplied andthe volume flow rate of the vapor being recovered to be kept equal inall circumstances because:

inevitable internal leaks exist in rotary vapor pumps, the size of theleaks increasing with wear and with the upstream-downstream pressuredifference that such pumps must generate to cause the vapor to flow andto be transferred to the storage tank;

it is impossible to be aware at all times of the states of the vaporrecovery pipes, especially those integrated in the fuel-dispensing hosepipes, it being possible for their headloss coefficients to varyconsiderably over time; and

the pressure in the storage tank varies.

One known way of avoiding those drawbacks consists in inserting into therecovery pipe a gas flow meter optionally associated with a pressuresensor so as to servo-control the speed of rotation of the vapor pump orthe opening of a valve so as to obtain the desired vapor flow rate.

However, this method makes it necessary to implement a rapid-responseservo-control loop receiving flow-rate and pressure information suppliedby the measurement sensors and acting on the actuator after comparingthe information with a reference value that can itself vary very rapidlyover time with the flow rate of liquid fuel as controlled by the user.

OBJECTS AND SUMMARY OF THE INVENTION

The technical problem to be solved by the present invention is toprovide a recovery system for recovering hydrocarbon vapor given off bymotor vehicle tanks while they are being filled with liquid fuel via atleast one dispensing pipe, which system makes it possible to obtain goodproportionality between the flow rate of vapor to be recovered and thespeed of rotation of the vapor pump, such as a pump having vanes, gears,or rotary pistons, without having to use a complex and costlyservo-control loop such as the loop described above with reference tothe state of the art.

According to the present invention, the solution to the technicalproblem posed consists in that said system comprises a recovery pipeassociated with said dispensing pipe, a power pump placed on saidrecovery pipe, and a metering pump driven by a motor at an angularvelocity that is substantially proportional to the volume flow rate ofliquid fuel delivered by the dispensing pipe, said metering pump beingmounted in series with said power pump via a differential pressureexpander-regulator suitable for maintaining a pressure difference thatis small between the outlet and the inlet of the metering pump so as toobtain a vapor volume flow rate that is substantially proportional tothe angular velocity imparted by said motor.

In this way, the accuracy of the metering pump is made to comply withthe regulations by the presence across its terminals of theexpander-regulator which constrains said pump to operate under adifferential pressure that is very small and if possible zero, theinternal leaks becoming negligible. Since the volume flow rate of vaporis proportional to the speed of rotation of the motor, which speed isitself proportional to the volume flow rate of liquid fuel, it can thenbe understood that it is possible to obtain the desired equality betweenthe flow rate of vapor and the flow rate of liquid, regardless of thedelivery flow rate, the nature of the gas, the state of wear of thepump, or the flow resistance of the recovery pipe.

Under these operating conditions, the metering pump can supply limitedpower only for causing the vapor to flow. For this purpose, it must beassisted by the power pump, that is capable of providing at least themaximum demanded flow rate under any circumstances.

The metering pump is thus capable of metering the volumes of vaporpassing through the entire recovery system, and of playing a partcomparable to that of a flow-rate regulating valve but with metering ofmuch higher quality as a result of the absence of expansion.

A fraction of the power supplied by the motor for driving the meteringpump serves to overcome the mechanical friction of the pump, theremainder of said power serving to cause the recovered vapor to flow andto be transferred under a very low differential pressure, and thereforeat very low power.

Two embodiments of the recovery system of the invention may beconsidered.

In a first embodiment, said metering pump is mounted in series with anddownstream from the power pump. Optionally a pressure regulator may bedisposed downstream from the metering pump and from said differentialpressure expander-regulator.

In a second embodiment, said metering pump is mounted in series with andupstream from the power pump, an expander being disposed upstream fromthe metering pump and from said differential pressureexpander-regulator.

The latter embodiment offers the advantage that, with there being aplurality of said dispensing pipes, the power pump is common and isplaced on a common portion that is common to the respective recoverypipes, downstream from said differential pressure expander-regulators.

Finally, a preferred configuration of the recovery system of theinvention leading to an implementation that is simple and cheap consistsin that said power pump and said metering pump are driven by a commonmotor at an angular velocity that is substantially proportional to saidvolume flow rate of delivered liquid fuel, the power pump having a cyclevolume that is greater than the cycle volume of the metering pump.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and how it may be implemented can be well understood fromthe following description given by way of non-limiting example and withreference to the accompanying drawings, in which:

FIG. 1 is a diagram of a first embodiment of the recovery system of theinvention;

FIG. 2 is a diagram of a second embodiment of the recovery system of theinvention;

FIG. 3 is a diagram of the recovering system shown in FIG. 2, havingmultiple dispensing pipes and multiple recovery pipes; and

FIG. 4 is a diagram of a variant embodiment of the recovery system shownin FIG. 1.

MORE DETAILED DESCRIPTION

FIG. 1 shows a system for recovering hydrocarbon vapor given off bymotor vehicle tanks while they are being filled with liquid fuel via adispensing pipe 19. The upstream end of said dispensing pipe 19 isimmersed in a storage tank 1 for storing liquid fuel which is sucked upby a pump 2 delivering a volume flow rate Q_(v). The liquid fuel passesthrough a measurer 3 placed in the dispenser and suitable for causing apin to rotate at an angular velocity substantially proportional toQ_(v). A pulse generator 4, such as an optical disk, coupled to themeasurer 3, supplies pulses whose frequency N is proportional to thevolume flow rate.

At the outlet from the measurer 3, the dispensing pipe 19 takes the formof a hose pipe terminated by a gun 5 which is of the vapor recovery typein that it is equipped with a sleeve (not shown) serving to prevent thehydrocarbon vapor given off by the vehicle tank as it fills with fuelfrom dissipating into the atmosphere. Said gun is the starting point ofa recovery pipe 6 on which a power pump 7 is placed that may be ofarbitrary type, e.g. a vane pump, as shown in FIG. 1.

Said power pump 7 is actuated by a motor 8 having an angular velocity Ωsuch as to supply at any time almost all of the power necessary forpumping the vapor to be recovered, without necessarily having goodmetering quality as regards its ratio of flow rate to angular velocityΩ.

If the cycle volume of the pump 7 is V, the angular velocity Ω ismaintained at a value such that:

    ρVΩ>ρ.sub.0 q.sub.v0

where ρ₀ and ρ are respectively the density of the hydrocarbon vapor atatmospheric pressure p₀, and said density at the inlet of the pump 7,and q_(v0) is the flow rate of the vapor at atmospheric pressure, itbeing desirable for this flow rate to be made equal to the flow rateQ_(v) of liquid fuel in the dispensing pipe 19.

Downstream from the power pump 7 a metering pump 10 of the positivedisplacement type is mounted in series, which pump is driven by a motor11 controlled by an amplifier 14 suitable for transforming the pulsesignals output by the generator 4 into power signals capable of poweringthe motor 11, e.g. of the stepper type, and of causing it to rotate atan angular velocity ω that is therefore proportional to the volume flowrate Q_(v) of the liquid fuel delivered by the dispensing pipe 19.

As shown in FIG. 1, the metering pump 10 is mounted in series with anddownstream from the power pump 7 via a differential pressureexpander-regulator 9 of a conventional model that receives from saidpower pump 7 hydrocarbon vapor under a pressure P higher than theinitial pressure p₀. After the vapor has expanded from chamber A tochamber B, the differential pressure Δp between the input and the outputof the metering pump 10 is maintained at a value ±ε that is very small.More precisely, the pressure in chamber B of the expander-regulator 9takes values lying in the range p'-ε to p'+ε, where p' is the pressureat which the metering pump 10 returns vapor to the storage tank 1 insidewhich the pressure is p', ignoring downstream headloss.

When only the power pump 7 is actuated by the motor 8, extra pressureP>p₀ builds up in chamber A of the expander-regulator 9. Since themetering pump 10 is stopped, thereby slowing down the flow of vapor, thepressure rises in chamber B and the valve of the expander-regulatorcloses immediately, if it was not already fully closed. Starting up thepump 10 enables vapor in B to be removed and the pressure therein fallsto a value p'±ε in the vicinity of the reference pressure p' of theregulator, which reference pressure is the pressure continuouslyexisting in the chamber C that is connected to downstream of themetering pump 10, and that is separated from chamber B by the regulatingmembrane.

An adjustment spring 12 enables the pressures upstream from anddownstream from the pump 10 to be adjusted when necessary.

Since the power pump 7 can supply a mass flow rate that is higher thanthat removed by the metering pump 10, chamber A of theexpander-regulator 9 is always under pressure P>p₀, and the reduction ofP to p'±ε can always take place between chambers A and B via theregulating valve.

Thus, since the metering pump 10 works under very similar upstream anddownstream pressures, it acquires very good metering quality, becauseinternal leaks become negligible.

With the mass flow rate of gas to be sucked up being ρ₀ q_(v0), themetering pump 10 must remove and transfer the same quantity p'vω, wherev is its cycle volume and ρ' is the density of the vapor under thepressure p' that is common to upstream and to downstream from the pump,hence:

    ε=ρ.sub.0 q.sub.v0 /ρ'v=ρ.sub.0 Q.sub.v /ρ'v

If, as is generally the case, ρ' is very close to ρ₀, to within 10⁻²,the motor 11 merely needs to be rotated at the velocity ω=Q_(v) /v.

If the pipe 13 leading to the storage tank 1 is very resistant to flow,or if the tank 1 is maintained under extra pressure, it is merelynecessary, as shown in FIG. 4, to dispose a pressure regulator 18 onsaid pipe 13 downstream from the metering pump 10 and from theexpander-regulator 9. The working pressure of the pump 10 is thenmaintained at a value in the vicinity of p", which value is higher thanthat necessary for removing vapor to the tank 1. Since in this case thepump 10 operates under the pressure p" to which a vapor density p"corresponds, the angular velocity ω of the pump 10 is adjusted to:

    ω=ρ.sub.0 q.sub.v0 /ρ"v=p.sub.0 Q.sub.v /ρ"v≈p.sub.0 Q.sub.v /p"v

proportional to N if p₀, p", and v are fixed.

The embodiment shown in FIG. 2 differs from the embodiment describedabove with reference to FIG. 1 in that the metering pump 10 operating atΔp≈0 is mounted in series upstream from the power pump 7. It should benoted that, in this case, both the pump 10 and the expander-regulator 9work under a pressure P₁ that is less than p₀, and that is created bythe suction of the power pump 7, hence the presence upstream from thepump 10 and from the expander-regulator 9 of an expander 15 for reducingthe pressure to the value P₁.

Furthermore, the power pump 7 situated downstream always supplies enoughpressure to deliver vapor when the pipe 13 downstream therefrom opposesresistance to flow.

Once again:

    ρ.sub.0 q.sub.v0 =ωvρ.sub.1

where ρ₁ is the density of the vapor at the regulated pressure p₁,hence:

    ω=ρ.sub.0 q.sub.v0 /ρ.sub.1 v≈p.sub.0 Q.sub.v /p.sub.1 v

proportional to N if p₀ and p1 are fixed.

The vapor recovery system shown in FIG. 2 may be easily extended to aplurality of dispensing pipes.

As shown in FIG. 3, in which there are two dispensing pipes 19a, 19bterminated by respective guns 5a, 5b, there is a single power pump 7placed on a common portion 6 that is common to the recovery pipes 6a, 6bdownstream from the differential pressure expander-regulators 9a, 9b.

The diagram in FIG. 4 shows a variant embodiment of the recovery systemof the invention, in which variant embodiment the power pump 7 and themetering pup 10 are driven by a common motor 17 at an angular velocity ωthat is substantially proportional to the volume flow rate Q_(v) of thedelivered liquid fuel.

The outlet of the power pump 7 of cycle volume V is connected to chamberA of the expander-regulator 9. Chambers B and C are connectedrespectively upstream from and downstream from the pump 10 which thusworks at a very small differential pressure, and whose cycle volume v isless than V.

Once again:

    ωvρ"=ρ.sub.0 q.sub.v0

    i.e.

    ω=ρ.sub.0 q.sub.v0 /p"v≈p.sub.0 Q.sub.v /p"v

proportional to N if p₀ and p" are fixed.

Since, under these conditions, the metering pump 10 does not remove asmuch vapor as the pump 7 is capable of supplying, extra pressure P>p₀builds up in chamber A of the expander-regulator 9, and its valve canreduce the pressure of the vapor from P to p" in the vicinity of theinlet of the metering pump 10, which operates under the pressure p" withan inlet/outlet differential pressure in the vicinity of zero.

Another advantageous characteristic of the vapor recovery system of theinvention is that it is possible to perform a dry test without making asystematic error due to the differences in viscosity and in molecularmass between air and the mixture of air and of hydrocarbon vapor.

Viscosity and density no longer play any part in establishing the flowrate since said flow rate is determined by the metering pump alone whichmeters out volumes at constant pressure. Neither do they play any partin internal leaks, since such leaks become negligible.

I claim:
 1. A recovery system for recovering hydrocarbon vapor given offby motor vehicle tanks while they are being filled with liquid fuel viaat least one dispensing pipe, said recovery system comprising a recoverypipe associated with said dispensing pipe, a power pump placed on saidrecovery pipe, and a metering pump driven by a motor at an angularvelocity that is substantially proportional to the volume flow rate ofliquid fuel delivered by the dispensing pipe, said metering pump beingmounted in series with said power pump via a differential pressureexpander-regulator suitable for maintaining a pressure difference thatis small between the outlet and the inlet of the metering pump so as toobtain a vapor volume flow rate that is substantially proportional tothe angular velocity imparted by said motor.
 2. A recovery systemaccording to claim 1, wherein said power pump and said metering pump aredriven by a common motor at an angular velocity that is substantiallyproportional to said volume flow rate of delivered liquid fuel, thepower pump having a cycle volume that is greater than the cycle volumeof the metering pump.
 3. A recovery system according to claim 1, whereinsaid metering pump is mounted in series with and downstream from thepower pump.
 4. A recovery system according to claim 3, wherein apressure regulator is disposed downstream from the metering pump andfrom said differential pressure expander-regulator.
 5. A recovery systemaccording to claim 1, wherein said metering pump is mounted in serieswith and upstream from the power pump, an expander being disposedupstream from the metering pump and from said differential pressureexpander-regulator.
 6. A recovery system according to claim 5, wherein,with there being a plurality of said dispensing pipes, the power pump iscommon and is placed on a common portion that is common to therespective recovery pipes, downstream from said differential pressureexpander-regulators.